This invention relates generally to angle measurement devices and more particularly to such devices in which the measurement is made with respect to gravity using either a capacitive or resistive sensor.
Several angle measurement devices are known in the prior art, of which U.S. Pat. Nos. 4,503,622, 4,707,927, and 4,547,972 are exemplary. These prior art angle measurement devices typically employ an angle sensor that is physically separated from the electronics employed in the devices. The angle sensors are generally of the resistive type employing a potentiometer having an attached pendulum to reference gravity or of the capacitive type employing a liquid-filled glass vial. The resistive type prior art sensors suffer inaccuracies due to friction in the pivot joint, while the capacitive type sensors employing glass vials are expensive to produce. The physical separation of either type of sensor from its associated electronics is disadvantageous, particularly in the case of capacitive sensors in which the length of leads connecting the electronics to a remotely located sensor or the movement of those leads affects the capacitance of the sensor.
U.S. Pat. No. 4,644,662 is directed to a capacitive type angle sensor that employs a teflon coating on the sensor plates to insulate them from the fluid contained within the sensor and to serve as part of the sealing mechanism. These sensors are also taught to be remotely mounted. The coating on the sensor plates, in conjunction with rings machined into the sensor housing, serves to seal the fluid inside the sensor. However, this sealing arrangement is prone to leakage if the housing screws are not precisely torqued. In addition, the maximum temperature at which these prior art sensors can operate is limited because at higher temperatures the vapor pressure inside the fluid vessel increases to a point beyond which the sealing rings are not effective, thereby permitting the undesirable escape loss of fluid.
It is therefore a principal object of the present invention to provide an improved angle measurement device in which the angle sensor and associated electronics are an integral unit, thereby eliminating the problems associated with prior art angle measurement devices in which those components are physically separated.
It is a further object of the present invention to provide an improved angle measurement device in which the containment of the sensor fluid is accomplished in a way that permits operation at higher temperatures than was previously possible.
It is yet another object of the present invention to provide an improved angle measurement device in which the user may remotely control the electronic gain of the device.
These and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a printed circuit board having electronic components mounted on the front surface thereof and having a pair of sensor plates etched into the back surface thereof. Containment of the sensor fluid adjacent the sensor plates is accomplished by soldering a sensor cover to an etched sealing ring on the back of the circuit board. The sensor plates etched into the back of the printed circuit board, in combination with the sensor fluid, act as variable resistors whose resistance changes as a function of their angle of rotation from a reference position. An output signal across these variable resistors is in or out of phase with an applied reference signal. The output signal is demodulated with the same reference signal and then filtered to a DC level that corresponds to the angle of rotation. An output stage serves as a driver and also as a means for permitting the user to remotely control the electronic gain of the angle measurement device.
In accordance with an alternative embodiment of the present invention, a capacitive type sensor may be provided in which a paraylene or solder mask coating covers the sensor plates etched into the back surface of the printed circuit board, but does not extend into the area of the etched sealing ring. This is in contrast to the capacitive type sensor taught in U.S. Pat. No. 4,644,662, mentioned above, in which a teflon coating covers the sensor plates and also acts to seal the resistive fluid within the sensor.